1. Field of the Invention
The present invention relates to a stator structure of an electric motor and in particular to a structure of a stator with a slot-less core.
2. Description of the Related Art
In various precision instruments such as optical instruments, electronic instruments and the like, components need to have machining precision in the order of nanometer to meet demands for high precision, high density and high integration. Machine tools, steppers and electron beam delineation devices for machining those components with high precision are required to have resolution of extremely high precision. In those machining and manufacturing devices, positioning is performed by a positioning device, and the position control by the positioning device is performed generally with a rotary servomotor or a linear motor which is controlled by a CNC. Thus, in order to improve the machining precision of components, it is necessary to control the rotary servomotor or the linear motor with high precision, so that electric current for such precise control is rendered very small. Therefore, even minute electromagnetic noise may influence the electric current for the precise control greatly.
In general, the rotary servomotor or the linear motor has torque ripple which is a significant factor in precision of the motor, and efforts to improve precision of the motor have been made by reducing the torque ripple.
Torque ripple can be broadly classified into machine-structural torque ripple and electromagnetic torque ripple. For example, in the rotary servomotor, frictional resistance acting on bearings for a rotor shaft causes the machine-structural torque ripple, and magnetic distortion produced between a rotor and a stator causes the electromagnetic torque ripple.
In order to reduce the machine-structural torque ripple, it has been proposed to support a shaft in a non-contact manner with a pneumatic bearing or a magnetic bearing to thereby reduce the frictional resistance. In addition, a stator of slot-less structure is proposed for suppressing cogging torque due to slots formed in a stator core in order to reduce the electromagnetic torque ripple. In view that a position precision and a manner of winding are important factors in deciding electromagnetic action in the slot-less stator core, it has been proposed to form windings in a toroidal shape and in a regular winding manner so to secure position precision of the windings in U.S. patent application Ser. No. 09/327,471 by inventors of the present invention.
Such a stator structure with toroidal windings is shown in FIG. 10. In FIG. 10, winding segments 100 are arranged at appropriate positions with appropriate intervals therebetween on an annular stator core 102 according to a type of the motor and the number of poles. A printed board 101 is arranged on both sides of the stator core 102 and a wire 104 is wound around the printed board 101 and the stator core 102 in a regular winding manner to form the winding segment 100.
A distribution pattern layer 113 is arranged at an outermost portion of the printed board 101 and a winding start point 106 and a winding end point 107 of the wire 104 are connected to the distribution pattern layer 113 to constitute an electric circuit including the wire 104 of each winding segment 100. This structure reduces so-called lugs of winding which are part of the winding projecting from the stator core in an axial direction of the motor and electromagnetically not contributing to rotational force of the rotor, and also enables to design a compact motor. Further, since the toroidal windings formed in the regular winding manner tie up the stator core and the printed board, the strong structure of reducing peel-off or slip-off of the winding is obtained. Furthermore, since an edge of the stator core which exerts bending stress to the winding and causes damage to coating of the winding is insulated by the printed board, the winding is prevented from being grounded through the stator core. In addition, the damage of the wire of the winding can be prevented by setting radius of curvature of the edges of the printed board to be relatively large.
Since the windings are formed very close to the printed board in this stator structure where the windings are formed on the printed board arranged on the stator core, the electromagnetic noise caused by electric current flowing in the distribution pattern layer gives undesirable influence on the current flowing in the windings and vise versa by the interactive electromagnetism.
FIG. 11 is a partially sectional view of the stator 103. As shown in FIG. 11, the stator core 102, the printed board 101 and the windings 104 are successively laminated to form the stator 103. The printed board 101 has the distribution pattern layer 113 arranged on a resin layer 111, which is connected to the winding 104 via a connecting element 105.
Exciting current flows in each phase (U, V, and W phases) of the winding 104 through the distribution pattern layer 113 according to the exciting control and the electromagnetic nose is produced in the winding 104 and the distribution pattern layer 113. Since the winding 104 and the distribution pattern layer 113 are arranged adjacent to each other, the produced noise influences the current flowing in these elements. As stated above, since the electric current for precise control of the motor is very minute, even very minute electromagnetic noise between the distribution pattern layer and the windings may influence the electric current control greatly.